The General Court of the European Union overturned the emissions compliance levels under the new Real Driving Emissions (RDE) regulation in a verdict announced on 13 December. On the surface of it, this may look like a victory for cities wanting to be tougher on emissions of nitrogen oxides (NOx) from passenger cars and vans. In reality, it has the potential to further complicate the Euro 6 regulatory stage and thereby create the unintended consequence of making it even less usable for urban vehicle policy.

While the European Commission did not seek to increase the headline 80 mg/km emissions limit for diesel vehicles, which must still be adhered to in the laboratory test, they granted “Conformity Factors” that in effect did increase the limits for the harder, on-road part of the test. As a result, all diesels vehicles only had to meet 168 mg/km by September 2019, falling to 114 mg/km by January 2021. This was in practice a large increase in the permissible emissions limit.

The Court verdict suggests that vehicles already certified under Real Driving Emissions (that starts with the stage known as “Euro 6d-temp”) will remain compliant, as will vehicles certified for up to 14 further months in the future, depending on whether the Commission appeals and the speed with which replacement legislation is brought forward.

But what is this likely to mean in practice?

Let us assume that there is no appeal and no new legislation, meaning vehicles must meet 80 mg/km on the RDE test. Taking a sample of 30 RDE-certified diesel cars tested by Emissions Analytics on its independent EQUA Index test, we conclude that up to 90% of the vehicles would still be compliant. Although the test does not include cold start in its ratings, overall it remains a good proxy for RDE compliance. Furthermore, the average NOx on a combined cycle of these still-compliant vehicles is just 50 mg/km, well below the certification standard. Of the remaining 10%, they all come from the same OEM, Honda, which would in this scenario need to make changes.

This suggests the impact would be low. However, RDE does not become mandatory for all vehicles sold until September 2019. Therefore, it is likely that this conclusion is flattered due to a self-selecting sample of the best performing cars. If we look at the wider population of pre-RDE Euro 6 diesel cars, we may have a proxy for the challenge to each manufacturer of no Conformity Factors.

The table below shows the proportion of pre-RDE Euro 6 diesels from a range of manufacturer groups that meet various emissions levels, on our real-world scale. For example, 33% of all BMW/Mini Euro 6 diesel vehicles tested by Emissions Analytics achieve a rating of ‘A’

The ‘A’ rating is equivalent to 80 mg/km; ‘A’ to ‘C’ is equivalent to 180 mg/km (the Euro 5 level, close to the 2.1 Conformity Factor level); ‘A’ to ‘D’ is equivalent to 250 mg/km (Euro 4).

Approximately, the Conformity Factors allowed ratings of up to ‘C’, but with this annulled, ‘A’ ratings would need to be achieved. The table lists the five manufacturer groups that would be best placed to meet this requirement – assuming their pre-RDE performance is a proxy indicator of how close they are to achieving 80 mg/km across their whole range. VW Group, therefore, is best placed overall; Jaguar Land Rover has made the most rapid advances in the last year.

The final column of the table indicates the proportion that meet ‘A’ to ‘D’. This is relevant to the 270 mg/km maximum limit proposed in an agreement between the German government and cities, to be judged in real-world conditions. The ‘D’ rating is a near equivalent to this level. While the 270 mg/km limit is currently only proposed to apply to Euro 4 and 5 diesels, in time it could be extended to Euro 6 vehicles. If this were to happen, the table shows what proportion of these manufacturers’ vehicles may be restricted from the 14 German cities cited. Therefore, VW, BMW and JLR would be the least affected of all. In addition, this is before the benefit of the fixes and retrofits that have been, or may be, actioned, which would further reduce the restrictions. In contrast, there are manufacturers that could face having all their Euro 6 diesels restricted.

Overall, this may give reassurance to some, but there is a wider risk. The annulment of these Conformity Factors further confuses what “Euro 6” means as a label of performance. RDE was meant to be a discontinuity with the past, failed regulation. It had two levels already – two Conformity Factors – but the effect of the Court judgement could lead a change in the gradations or, potentially, more gradations. The Euro 6 label has limited informational content already, but the effect may be to cloud what RDE means, causing further consumer confusion, which would not be to the advantage of the car market.

The Court judgement is witness to the growing power of cities in determining vehicle emissions policy, even if they sometimes demonstrate an unresolved tension between whether air quality improvement or greenhouse gas emissions reduction is the higher priority. What the Court judgement may do is bring into starker relief the difference between the best and worst performing vehicles, which would pave the way for more efficient solutions to the urban air quality challenge.